Diesel engines emit a hazardous, sooty exhaust that can be rendered less hazardous by using diesel particulate filters. Known diesel particulate filters, however, unduly increase fuel consumption and tend to have poor durability.
Known diesel particulate filters can be divided into four categories or types, wherein the first type is characterized by having a ceramic wallflow monolith, the second type having a rigid ceramic foam, the third type having a mass of fibrous filter material, and the fourth type having one or more hollow, perforated, metal tubes. Each filter type can have an elongated tubular casing adapted for connection to an exhaust system. Further, each filter type can be periodically regenerated during use by burning off the collected particulate matter.
An example of a first-type diesel particulate filter is shown in U.S. Pat. No. 4,276,071 (Outland).
Offenlegungsschrift No. DE 38 06 131 (Giebling), laid open Aug. 31, 1989, describes a method of making a modified first-type diesel particulate filter, wherein a paste-like ceramic structural material bearing a mesh including parallel, spaced filaments is wound up into a roll that is inserted into a tubular casing and then fired to produce a porous ceramic soot-filter body.
Examples of second-type diesel particulate filters are shown in U.S. Pat. Nos. 4,264,346 (Mann) and 4,813,231 (Bykowski). A problem common to both the first and second type filters is that due to their rigid nature, they are prone to cracking when subjected to thermal stresses and vibrations, such as, for example, those encountered when the filters are used in automobiles or other vehicles.
One example of a third-type diesel particulate filter is disclosed in Offenlegungsschrift No. 35 45 762 (Brich) laid open Feb. 7, 1987, wherein woven mats are placed in a cylindrical housing so that they are coaxial with the centerline of the housing. The housing appears to have an axial gas inlet and an axial gas outlet. The exhaust gas appears to pass radially through the woven mats. In one embodiment, ceramic fiber mats are rolled up tightly and fill the entire cross-section of the housing. This embodiment optionally has a metallic web rolled together in a spiral fashion with the ceramic fiber mat so that they become enmeshed around the axis.
Offenlegungsschrift No. DE 38 01 634 (Stopler et al.), laid open Aug. 3, 1989, discloses a third-type diesel filter wherein several layers of cylindrical filter elements are arranged in a housing unit between an inlet and outlet funnel to be radially traversed by the exhaust gases, wherein the filter elements have an essentially circular or oval cross-section with an equidistant radial spacing under the formation of inlet and outlet channels, and wherein each filter element comprises a gas-permeable filter-body of wire-mesh, ceramic fibers, or a combination thereof, enclosed at both sides by a gas-permeable metallic cover-sheet. Further, by using spacer elements, the filter-body can be wound to form a wrapper with a spiral inlet and outlet channel, each closed at the opposite side, wherein the enclosing covers formed by perforated metallic sheets or metallic weaves are provided with a catalytic coating for converting the gaseous pollutants.
An example of a fourth-type diesel particulate filter is shown in U.S. Pat. No. 4,324,572 (Erdmannsdorfer), wherein a plurality of parallel, spaced, rigid, perforated, hollow tubes (designated "support pipes") extend substantially the length of the casing. Threads of spun silicon dioxide or silica fiber are wound on each of the tubes to provide filtering elements. Each tube is plugged at the inlet, while channels between the tubes are blocked by a wall at the outlet, forcing the exhaust to pass through the filtering elements. The threads are said to preferably be cross-wound to allow oxygen to reach the soot deposits during periodic burn-off of collected particulate matter.
Offenlegungsschrift No. DE 37 31 766 (Buck), laid open Mar. 30, 1989, discloses another fourth-type diesel particulate filter having an enclosure with an inlet and outlet connection pipe, wherein ceramic fibers or yarns are placed inside the enclosure.
U.S. Pat. No. 4,576,799 (Worner), discloses yet another fourth-type diesel particulate filter comprising a plurality of parallel, spaced, rigid, perforated, hollow tubes substantially filling an elongated casing radially and concentrically. Each of the tubes consists of a filtering element sandwiched between an inner perforated shell and an outer perforated wall. In one embodiment, there is a barrier between adjacent tubes so that exhaust entering between two of the tubes passes radially outwardly through the filtering element before exiting through the space between the outer of those two tubes and the next barrier. In another embodiment, there are two tubes between adjacent barriers, such that the exhaust passes through the filtering elements of both tubes before reaching the outlet.
A fourth-type diesel particulate filter is also disclosed in Offenlegungsschrift No. DE 39 10 554 (Engeler et al.), laid open Oct. 19, 1989, wherein the filter comprises four concentric, rigid, hollow, perforated tubes extending substantially the length of the casing, with each space between adjacent tubes being blocked at only one end of the tubes so that exhaust entering an unblocked space passes radially inwardly or outwardly through the perforated walls of the tubes.
U.K. Pat. Appln. No. GB 2,193,656 (Henkel), published Feb. 17, 1988, discloses a fourth-type diesel particulate filter that has only one perforated hollow tube, wherein the tube is wrapped with two filtering layers, first a fine-pore woven fabric of quartz glass or ceramic fibers and then an elastic wide-mesh fabric or fleece.
A common problem associated with a fourth-type of diesel particulate filter that employs short or staple fibrous filter material is that it tends to experience fiber shedding resulting in an undesirably short life.